text:
What separation and information does ATC provide in different airspace classes?
I am busy studying radio for my PPL in South Africa, but I'm struggling to understand the ICAO definitions of the different airspace classes, and how ATC separates traffic in them.
I understand that the difference between airspace classes is based largely on which flights are "controlled" or "separated" by ATC, and which ones are only getting "traffic information". But I don't understand exactly what the difference is.
I fly a rotor-wing at a class D airport. Under VFR, we should only receive "traffic information", correct? But in reality, we still have to request ATC's permission to take off etc. and they can instruct us to hold, give way and so on. Doesn't that mean that we are in fact being "separated" from other traffic?
And theoretically, if IFR traffic is controlled by ATC in class D but VFR is not, and IFR is not separated from VFR, how should conflicting traffic avoid each other? If an IFR flight sees a VFR one in its way, who should do what? (Practically, the pilots will obviously maneuver to avoid each other, but what does the theory say?)
Finally, since IFR flights only get "traffic information" about VFR flights but not "separation", what happens if a VFR flight gets in front of an IFR one on final approach to land? Won't ATC tell the IFR flight to go around, or will they do nothing because they don't provide "separation"?

Welcome to aviation.SE! Which country are you in? This question covers the general concepts for the US, if that's where you are.

Thanks for the fast response! I am in South Africa, however my confusion still lies in understanding the ICAO definitions for the airspaces, specifically the practical difference between "controlled" / "separated" and "traffic information only". Thanks, I have read through that question before posting, but am looking for a more practical explanation, from a pilots perspective?

I edited your question to try to simplify it a little and include some information you put in comments. If I've overdone it or distorted your question, please just rollback or re-edit.

@Dane Remember to mark an answer, if you have an answer, to close the question.

Different countries have different meanings to the classification of Airspace. I am only looking at the controlled airspace for ICAO.
Controlled airspace is an airspace that an aircraft would need a clearance to enter, this doesn't mean all aircraft need a clearance to enter just some aircraft.
Classes of Airspace
Class A is for IFR flights only, all aircraft are separated from all aircraft and are known to Air Traffic Control (ATC).
Class B is like Class A but with VFR. IFR and VFR are cleared to enter, all aircraft are separated from all aircraft and are known to ATC.
Class C is where the major confusion on separation usually begins. IFR and VFR are cleared to enter, all flights are provided an ATC service. IFR flights are separated from IFR and VFR flights, but VFR flights are only separated from IFR flights. VFR aircraft are only passed traffic on other VFR aircraft. All aircraft are known to ATC. I will go into what they do when they get in conflict at the end.
Class D both IFR and VFR are cleared to enter, all aircraft are provided an ATC service. IFR aircraft are separated from IFR and passed Traffic on VFR flight. VFR flights are only passed traffic on other aircraft. All aircraft are known to ATC.
Class E IFR are cleared to enter and VFR are permitted to enter, only IFR aircraft are provided with an ATC service. IFR aircraft are separated from IFR aircraft and passed traffic on known VFR aircraft. VFR aircraft do not need a clearance and are not usually known to the Air Traffic Controller.
Class F No one is given a clearance. IFR aircraft are given traffic on IFR Aircraft and known VFR. But all pilots are responsible for their own separation.
Class G No one is given a clearance. Traffic is only passed where it is requested, and only on known traffic. Only those requesting traffic or are seen on a radar are known to ATC.
Answers to Specific Questions

I fly a rotor-wing at a class D airport. Under VFR, we should only receive "traffic information", correct?

Yes, that is correct.

But in reality, we still have to request ATC's permission to take off etc. and they can instruct us to hold, give way and so on. Doesn't that mean that we are in fact being "separated" from other traffic?

When talking about Class D airspace: you are only separated from other aircraft when on the runway; when you are in the air you are not being separated. You instead will be passed Traffic Information on other flights. You however are still subject to the conditions of your clearance (Route, Level, Holding Instructions, etc.).

And theoretically, if IFR traffic is controlled by ATC in class D but VFR is not, and IFR is not separated from VFR, how should conflicting traffic avoid each other? If an IFR flight sees a VFR one in its way, who should do what? (Practically, the pilots will obviously maneuver to avoid each other, but what does the theory say?)
Finally, since IFR flights only get "traffic information" about VFR flights but not "separation", what happens if a VFR flight gets in front of an IFR one on final approach to land? Won't ATC tell the IFR flight to go around, or will they do nothing because they don't provide "separation"?

If one or both aircraft are under a clearance the cleared aircraft will request an amended clearance to avoid the traffic:
ABC request 10NM right of track due traffic

or
ABC request descent 4500 due traffic

If there is not enough time for the amended clearance the Pilot in Command (PIC) can just break their clearance to avoid the traffic; either passing the aircraft on the right or changing levels. The PIC in some countries will be considered to have broken their clearance but it is usually only a slap on the wrist for letting it get that bad that you couldn't request the clearance change.
Reality can sometimes be a very different scenario. Some controller unofficially will separate VFR from IFR, but no one knows. Don't trust that this will happen.
Also controllers may give suggested amended clearances like:
ABC traffic XFV PC12 crossing track left to right at time 23,
6500 is available advise.

or
ABC traffic unknown VFR 25NM oposite direction 500 ft below unverified,
higher available advise

If the aircraft is not subject to a clearance then they can just move to avoid the traffic under the Rules of the Air.
In uncontrolled airspace it really is see and avoid. Because the Air traffic Controller may not have all the aircraft that are traffic for you. This also includes Class E airspace where the controller doesn't know about all the VFR aircraft.
Duty of care means that the controller may intervene if they think the pilots aren't separating themselves.
I work with Class D and C Towers with lots of VFR traffic. Because the traffic are aware of each other they usually request amended clearances well before the traffic is a problem.

Thanks! I think that the definition for Class D confuses me the most: IFR are separated from IFR, but not from VFR (which seems silly) - IFR won't collide with IFR, which is great, but VFR could theoretically collide with IFR, since "VFR is not separated from IFR by ATC". It is easy to understand the definitions as just words, but in practicality they sometimes don't seem to make sense to me, and in "reality" (as you have mentioned) I think that they are not necessarily implemented as per the wording (?).

It almost looks like ICAO defined the airspaces according to this neat pattern of decreasing "separation", but didn't think about how that actually makes sense in practice. I still think that it would be interesting to know, from an ATC point of view, how the airspaces differ for them, in terms of job / equipment requirements.

@Dane From an ATC Point of view (Which I am), we prefer to be control of everything because we are control freaks. Just because VFR are not separated from IFR doesn't mean it is unsafe and collision could occur. Traffic statements are able to inform pilots what they need to avoid themselves, Much better then not knowing anything like Class G.

In practice, that is what I would expect - you as the ATC are going to try and keep everyone separated from everyone, regardless of "airspace class". Perhaps it is best to just memorise the definitions for exam purposes, but forget about trying to understand any actual reasoning behind the definition for e.g. Class D, since in reality things will probably be implemented differently. Or maybe this will all make more sense if/when I have actual experience with something other than just VFR in Class D / G airspace.

@Dane You still need to know the books for reality, because a lot of it is run exactly as the books say, once you are doing it the books make a lot more sense.

You are confusing the terms controlled/uncontrolled with separation. Being controlled does not necessarily mean that separation is provided.
In a controlled airspace (for VFR, this would be class B, C, D - for IFR class A, B, C, D, E) you need a clearance to fly. That is, the controller needs to give you permission to be there. You cannot enter a controlled airspace without a clearance, and you cannot turn/change level without a clearance.
Let's take class D, since you bring it up yourself and it is actually quite an interesting example. In class D airspace, IFR and VFR is allowed, and both IFR and VFR need a clearance (meaning that both IFR and VFR are controlled). IFR flights will be separated from other IFR flights; it is the responsibility of the controller to do so. No separation is provided for VFR flights. If two flights - either two VFR flights or an IFR flight and a VFR flight - find themselves on converging courses, it is up to the pilots to avoid a collision in accordance with the rules of the air (e.g. give way on your right). Unlike uncontrolled airspace, the pilots will know of all other traffic around them, because the controller will give traffic information. The controller can do this, because every flight in the area needs a clearance to be there, and so the controller is aware of everyone. In practise, this means that pilots on an IFR flight do not need to constantly look out the window for other traffic, because the controller will inform them of it. But if they are informed of other traffic, they need to avoid it by means of the rules of the air.
Traffic information in uncontrolled airspace is different, because it cannot be guarenteed that the controller is aware of all traffic in the area, since flight do not need a clearance to be there.
With regards to runways, airspace classes do not apply. On a runway, there are only two states: controlled or uncontrolled. If an aerodrome is controlled, the tower controller will provide runway separation, regardless of flight rules (IFR/VFR).

So in e.g. Class D airspace, ATC can provide separation to VFR, but they do not have to? The difference lies in who is ultimately considered responsible for the separation? Is there any particular reasoning behind this - e.g. to reduce the workload (perhaps number of personnel required) for ATC, or based on their capabilities due to equipment / systems which they have available? Could it be said, in this example, that IFR are exactly told what to do / guided / actively controlled by ATC, whereas VFR can do what they like so long as they ask/get permission first (but may be controlled)?

@fooot You appear to be talking about FAA procedures, which this question is not about.

@Dane, No, ATC will not provide separation to VFR flights in class D airspace even if they could - because ATC is not responsible for separation in class D. Different airspace classes are used for areas with different traffic levels and complexity, and the cost/benefit of the service provided.

I think it would help if you explained the other classes, which would help to clarify the difference between "traffic information" and "separation", which the question seems to focusing on.

OK so perhaps it is important to emphasise (as I understand your answer), that runways / taxiways / etc on the airport itself are not part of the airspace, and that ATC will provide control (separation?) to all flights with regards to these? So whilst I am VFR in the circuit ATC cannot tell me what to do, only advise me of other traffic (and tell me where to report?), but when I come in to land then they can tell me what to do - whether I may land (or if I should go-around / orbit), where I may land, whether / where I may take off (and in what runway direction), etc?

@Dane In the circuit, ATC can tell you what to do, with the purpose of establishing runway separation - e.g. to ensure that they don't have two landings on top of each other. But they won't give you instructions with the purpose of establishing separation to other airborne traffic.


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Can noise from the passenger cabin be picked up by the cockpit voice recorder?
How sensitive is the cockpit voice recorder?  Can it pick up sounds from the passenger cabin?

It depends a bit. One of the microphones for the cockpit voice recorder is an "area microphone," which is designed to record whatever someone standing in the cockpit would hear (the others record from crew headsets and possibly the PA system). This area mic is continuously recording; it does not just turn on when there's more than some threshold of noise. If the noise in the cabin is loud enough to have been heard in the cockpit, it will be heard on that mic. If not, it won't.

I only have experience with general aviation (GA) aircraft, that being said, I bet the intercom in a commercial airliner is similar.
There is something in GA called "squelch" which is like a minimum volume limit for the microphone to turn on.  This makes it so background noise (such as the engine) doesn't make it into the intercom system.  If commercial airlines use this same technology it is very likely that any background noise from passengers is not going to get picked up by the intercom system, and it will not be recorded.
That being said, a commercial airline is much quieter than most GA aircraft, so they might not use squelch.  However, the microphones are still going to be pressed up against the pilot's lip, and any sound from the cabin that makes its way through the air, and cockpit door is going to be so quiet that it will be indistinguishable by the time it gets to the pilot's microphone.

The cockpit voice recorder gets input from pilot headsets, but it also has several microphones placed around the cockpit and those are unlikely to be squelched since they are for CVR only.

This answer is informational, unfortunately does not really answer the question any better than a guess.

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Why are listed times of useful consciousness so low?
I've been reading about Time of Useful Consciousness (TUC), and it says that at FL 180, the time of useful consciousness is 20 to 30 minutes at rest.
But I've been to a trip to Tibet, and we went on a bus trip around the places, and at one point the bus came to a mountain pass around 18,500 feet, and we all got off the bus, walked around, took pictures, and even ate lunch, so it was definitely not at rest. It was about 40 minutes to 1 hour, and I only felt slightly uncomfortable/out of breath.
According to the chart, I should have passed out, or at least become "useless". Why didn't this happen? Are the charts simply very conservative and apply to, say, unfit, crumbling, old pilots?

If you have the opportunity at such an altitude again, using just paper and pencil (no calculator), multiply two four-digit numbers. I tried this experiment after sitting at 14,000feet for an hour in a Cessna 310 while a friend was flying the airplane on oxygen. I did three of that kind of problem. When checked with a calculator, all three of my answers were wrong.

I was actually on my computer programming while on the bus (which stayed above 15,000 feet for about 2 hours), and I definitely felt pretty useful, if a bit easily-exhausted. I think it's just the acclimatization.

The Dunning–Kruger effect suggests that as your mental acuity declined, your effectiveness in assessing your own mental acuity probably suffered a corresponding decline.

In short: acclimatization, both chronic and acute.
During your Tibet-trip, you had probably spent days at high altitudes, allowing your body to acclimatize to the lower oxygen partial pressure (similar to how some elite endurance athletes prepare themselves for competition), by increasing your hemoglobin count (and other mechanisms), making you better prepared for the walk at 18,500 ft. This is an example of chronic acclimatization.
A normal sea-level dweller will not have had the benefit of the same acclimatization phase in a depressurization situation (albeit a pilot could probably have some degree of acclimatization from continuously being exposed to higher than sea-level cabin altitudes), and will hence have a shorter TUC. The onset of the depressurization will also play a big part in determining your TUC. A slow depressurization will allow for some acute acclimatization, whilst a rapid depressurization will allow for none. The shock of a rapid depressurization will also decrease your TUC.
The table on the Wikipedia page you quoted is the same (or very similar) to what you'll find in most Human Performance literature for airmen, and is, according the same Wikipedia page "average TUC for healthy, young military pilots".

That's probably true. I've stayed about a week above 10,000 feet (there's literally no place lower than 10,000 feet around) before getting to 18,500. I noticed that for about a week after returning to a sea-level place, I was able jog ridiculous distances without feeling tired.

Conversely when I was a teenager I walked to the top of a 10,000 feet peak (in France): I was panting at the top, and still panting after sitting there for 30 minutes.

@user54609: Whoa that's really cool... how far were you able to jog? I'm really curious now haha

@Mehrdad : Pretty unimpressively actually. It's just that before going to Tibet, I couldn't jog at all: I would go out of breath in about 2 minutes. I'm not exactly the most fit person on earth. After the trip, that became upwards of 15 minutes.

At lower altitudes, "useful" consciousness doesn't end when you are unable to act.  Rather, what happens is that your ability to make sound decisions and carry them out effectively degrades: for example, instead of responding to an alarm buzzer by figuring out what's causing it, you might simply hit the "silence" button, or you might be so fixated on keeping the airplane flying straight-and-level that you don't notice the alarm, or you might be unable to concentrate on either task long enough to do anything effectively.
I'm not familiar with aviation literature on the subject, but at Mauna Kea Observatories (14,000 feet), they found that providing supplemental oxygen to people working there greatly reduced the error rate.  See, for example, a Keck Observatory article on the subject, or a study on the effects of altitude for a proposed Chilean observatory that heavily references Mauna Kea.

After thinking for bit, perhaps this applies in a way in my case too. I am usually reasonably good with my DSLR, but all the photos I took from that pass were very poorly framed, poorly exposed, or even completely out of focus, and I didn't even notice until later.

Do you have a citation on the Mauna Kea claim?

@MarchHo, I've been trying to find it.  I seem to recall that it was an offline source, which makes searching hard.

try here: "Supplemental oxygen is also a good idea for visiting scientists and engineers who have to do other thought-intensive work while on the summit. Otherwise they could find themselves more prone to errors or just plain stumped by problems that would be easy to solve at lower elevation."

also, this: "at high altitude[,] accurate work can be done, particularly if detailed protocols are written out, but it takes longer and more concentration is required. Problem solving is particularly difficult and this is where errors are most likely. The principal way in which it is planned to respond to the problem of reduced mental ability is to provide an oxygen enriched atmosphere in those situations where it is required"

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Do pilots use electronic checklists?
I read that pilots often use printed checklists.
Is this a legal requirement, or are there also electronic versions available?

Yes they do, and according to the FAA they're better than paper ones. All sorts of aircraft can have electronic checklists, from large airliners to small Cessnas: modern avionics like the G1000 support checklists and the older but very common GNS430 does too.
In the US at least there's no legal requirement to use checklists in any form - electronic or paper - with some caveats:

They're considered a fundamental best practice in aviation, so if you don't use them then someone could argue that you're acting carelessly or recklessly
Incorrect - especially missing or incomplete - checklist use has been linked to many accidents (see this question too) so clearly it's in your own best interests to use them properly
If you're flying for an airline then your OpSpec probably requires use of checklists, in which case they're mandatory

As for mobile devices, there are already many EFB applications and standalone apps that support checklists. In some cases the app provides a template for a specific aircraft type, in others you have to enter it all yourself. There may be some copyright considerations, but third-party checklists are very widely available so obviously it isn't a huge issue.
Finally, it's worth noting that many pilots - especially those who own their own aircraft - customize checklists for their own needs.

Newer, more advanced, more expensive aircraft ever more come with computers that have checklists built in that can be displayed on a screen on the flight deck. Think the Boeing 777 or Airbus A380 for example.
This might be optional or standard depending on aircraft type.

Are there regulations preventing such things from being turned into software for mobile devices? I doubt it, long as the device itself is approved and the software can get approved for reliability and accuracy.

But you'd be unlikely to be able to make something like that unless you're working for or contracted by an aircraft manufacturer as you'd not have access to the data needed for the software to display. That data isn't static and is copyrighted (so you can't just take an old paper checklist and turn it into some html pages for example).
You'd have to have some way to update the checklists as new versions are released, and that'd mean having channels with the manufacturer to gain access to their data (which may not be available at all electronically, you'd likely not be able to get it except for the same aircraft that already have such systems built in, making the entire idea superfluous).

I doubt the POH/QRH would not be available as pdf or similar format and then all you need is a tablet with appropriate reader. It is still reasonable to have a printed copy in the cockpit somewhere in case battery goes flat or something though.

@JanHudec possibly, but the question was about custom software. Something like an EFB on your iphone.

@jwenting: EFB is a lot more than just checklists. Things like weight and balance calculator obviously need to be tested and approved. But I don't see that mentioned in the question, only checklists, and for those electronic version of the POH/QRH and appropriate viewer would be enough.


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What is the difference between active and passive sidestick?
What is difference between "active" and "passive" sidestick?
Which aircraft have active sidestick?

For references, I've used United States Patent Application 20080156939; BAE Systems Develops Civil Aircraft Active Control Stick; Active and passive side stick controllers in manual aircraft control; & the Side-stick wiki entry. The wiki is a general side-stick discussion listing (some) aircraft that have side-stick fly-by-wire (FBW) technology, but does not go into active vs. passive.
Generally speaking, if you have used a joystick on your PC to fly Microsoft's Flight Simulator, such as Microsoft's Force-Feedback Sidewinder Pro, you've used an active side-stick.
According to the Patent Application cited above, the patent is for an Active Side-Stick technology described as "pilot flight control stick haptic feedback mechanism provides variable force feedback to the pilot flight control stick." One of the 'issues' with the current passive side-stick technology (approved for use) used on aircraft such as Airbus' A380 is that there is no tactile feedback when using the side-sticks, such as a pilot would feel using an old-style column or yoke. In a Cessna 172, for instance, when you pull/push/turn the yoke, there is actual tactile feel, if you pull back strongly in-flight, you'll 'feel' the forces exerted against the airflow. Passive side-sticks have none of this 'feel'.
Active SS technology attempts to replicate the forces on the joystick.
Active SS technology is currently used on most miltary aircraft using side-sticks, but does not appear to have yet made its way into civilian airliners.
(If I've gotten stuff wrong, I'll remove/fix this answer, as I am not an airline pilot and have never had the opportunity to fly military hardware.)

As far as I can tell, the B777 has active control column (it is not side-stick, but it provides the feedback via the same electronic mechanism as the active side-sticks). On the other hand the passive side-stick in Airbus does exactly the right thing there. The side stick commands rate of pitch/roll and that's what you are getting feedback for.

A passive sidestick only provides back pressure forces when it is deflected, by mechanical phenomina such as springs, dampers, and friction. It is passive in that it cannot be made to move other than by pushing against it, it never moves by itself.
An active side stick has some element installed that enables movement of the stick other than by hand force: a motor for instance. This is technology that has been applied for over half a century in flight simulators.
The side sticks in the Cirrus SR22 are active side sticks, they are coupled directly to elevators and aileron and provide direct feedback of the aeroforces.
Image source
When I worked for a company making force feedback systems for flight simulators, I worked on a contract from Mercedes for making an active side-stick for a test car. Driving a car with a passive side stick results in Driver Induced Oscillations: command a left turn, the sideways acceleration applies force on your arm which moves the stick, you then correct etc etc.
An active stick allows acceleration feedback into the resistance force signals, so that the driving arm does not move but is held in position by a neutralising force, proportional to acceleration. The active 2-channel side stick was driven by electric motors with computer driven force resistance. The test drive on their circuit in Stuttgart was very enjoyable.


text:
Which computer controls auto thrust on an Airbus A320?
An Airbus A320 has seven computers that control flight surfaces:
two ELAC, three SEC, two FAC, and two AP (autopilot) computers.
Is there an separate computer that controls auto thrust, or is this controlled through one of the above?

More specifically on your question:
See diagram below:

In the autothrust mode (A/THR function active), the FMGC computes the thrust, which is limited to the value corresponding to the thrust lever position (unless the alpha-floor mode is activated).

So as you can see computers/controllers that control the A/THR are:

FCU
FMGC/S
EIU or EEC
FADEC
ADR

The **speed or Mach target may be:

‐ Selected on the FCU by the flight crew
‐ Managed by the FMGC.

EPR mode is the normal mode to control the thrust. The required EPR is set by controlling the fuel flow.
DSC-70-35-20 P 1/2
The FADEC has two mode of power setting, EPR and N1 modes.
The FADEC computes the command EPR as a function of :
‐ Thrust Lever Angle (TLA)
‐ Altitude
‐ Mach number
‐ Ambient temperature
‐ Air inlet total temperature (T2) ‐ Service bleed.
Arming conditions of the A/THR are numerous. The following is a list of the most important ones:
‐ One FMGC operative
‐ One FAC operative
‐ Two ADIRS operative
‐ Two FADECs operative
‐ One channel of the FCU operative
‐ One LGCIU operative
‐ A/THR is not manually disabled (instinctive disconnect pb has not been pressed for more than 15 s).

So to summarise:
  FMGS computes the THR and the FADEC computes EPR and through EIU/EEC and ECU commands the fuel flow to the engines in order
  to give the desired EPR.

The autothrust functions in the A320 are performed by the Flight Management and Guidance Computers, or FMGCs (what you call autopilot computers -- they indeed handle autopilot duties as well).

Is the FADEC part of the FMGC  or additional ?

@PeterBourne -- the FADECs are completely separate (1 primary/1 backup per engine)

The Flight management and guidance system (FMGC) performs the following functions:

Autopilot
Flight director
autothrust
flight management

The FMGS sends the thrust commands for the autothrust function to the ECU1/EEC1 to set the thrust command on the engine 1. ECU2/EEC2 to set the thrust command on the engine 2.


text:
Can a wooden/fabric cover design be built of metal?
I know this question is somewhat general and could vary from plane to plane so I will elaborate on the plane in question.
The specific plane I am curious about is the Mooney Mite which has a metal center section but wooden wings and wooden rear half and empennage which at the age most of these planes are is prone to issues. It seems like a great little plane.
My question is can one be built out of aluminum to the same specs (maybe with a bigger engine) and still fly properly or will the weight change (CG change) affect the plane too drastically?
Likewise could a Piper Cub or similar be made of all metal (again with a bit more power up front) and still be an effective plane?
For the record I do not currently have plans to build this plane (although it was available as a kit plane at some point in history. I am aware of the certification issues with making alterations like to an existing plane. I was asking more about if it was possible from an aeronautical standpoint.

I feel obliged to say that if you do decide it's a good idea to do this, check with the regulatory authorities first. It's quite likely that the major overhaul you're proposing could affect the airworthiness of the aircraft in question, and the CAA or FAA would have a lot to say about the result. In the UK, the CAA has a "owner builder" programme to help you get the result certified as airworthy again -- other countries might not be so accommodating.

Ed Swearingen used to say that it made little difference if you build an airplane from wood, aluminum or carbon epoxy - in the end the weight would not be too different if the parts are designed well.
Note that many aircraft in the 1930s used the combination of an aluminum frame and fabric covering. Especially the control surfaces of many aircraft, but also the fuselage, wing and tail surfaces of the Vickers Wellington were built this way. Also, many gliders before the advent of fiber-composite structures used a welded steel-tube frame covered with fabric for their fuselage. Since glueing works better with wood, the fabric cover needs to be sewed to the ribs and stringers, so suction forces will not lift it up over an extensive area.
Key is the limited dynamic pressure: If you do not increase operating speeds, exchanging wood for aluminum will be straightforward. Only the torsion box of the wing would need closer attention, since the low weight per area of plywood cannot be recreated with aluminum sheet - the wall thickness and the resulting buckling strength would be too low. You would be well advised to add more stiffeners and ribs!

I understand that more ribs etc would be needed I was not necessarily asking about a part for part wood for aluminum exchange but more a general shape/airfoil design exchange.

@Dave: All right, I just wanted to make sure that I am not misunderstood. If designed well, the metal structure should reproduce the wooden shape faithfully.

Yes the Mooney Mite can be constructed out of aluminium, but will have to be fully re-engineered. Wikipedia states:

The Mite is constructed mainly of fabric-covered wood, with a single spruce and plywood "D" wing spar.

Fabric cannot take any structural stresses, all are absorbed by the internal wooden frame construction. The fabric is not part of the structural design and is only there to provide an aerodynamic shape.
By contrast, aluminium aeroplane construction uses the Stressed Skin design, where the skin not only defines the external shape but also is loaded with compression, tension and twisting stresses. One could say that in an aluminium wing, the only function of the spar is to keep the skin apart so that it can do its structural work. In a wood/fabric construction the spare takes all of the load and is therefore of larger volume.
Stressed Skin construction is most effective if the skin sections are far apart: with larger aircraft. GA aircraft are generally of Stressed Skin construction nowadays so me might safely assume that the smaller Mite might be constructed in aluminium with little or no weight penalty. If properly re-engineered.

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text:
Can an aircraft be issued a standard airworthiness certificate after being operated under and experimental certificate?
14 CFR, §21.191 (b) states that experimental airworthiness certificates can be used when:

Showing compliance with regulations. Conducting flight tests and other operations to show compliance with the airworthiness regulations including flights to show compliance for issuance of type and supplemental type certificates (...)

Can the aircraft that is used for such testing be converted back to a standard airworthiness certificate after testing is complete?
Example scenario: Manufacturer A is developing a new autopilot to be offered as an after-market modification (along with the necessary STC). The manufacturer purchases an aircraft with a standard airworthiness certificate, installs the prototype avionics hardware, obtains an experimental certificate, and performs the necessary testing. Once complete, he manufacturer would like to return the test aircraft back to the condition it was before obtaining the experimental certificate in order to sell it with a standard airworthiness certificate.
Is that possible? If so, what would the manufacturer need to do in order to demonstrate that the aircraft was not operated outside of the limits of the standard airworthiness certificate while under the experimental certification? What FAA documentation exists that would provide guidance on this?

Not posting as an answer because I can't give you an answer to question 3, but yes an OEM will use an aircraft for test and later restore its AW cert and sell it to an operator. They won't do this with prototype a/c used for the original development testing, but will do so with production models that are transferred to the Experimental organization for cert testing of variants or systems.  All that's required is to make the a/c conform to requirements of its TC. That means restoring it to its certified config, and performing all maint/mods/tests as required.

Thank you @JohnK, makes sense. Is there any concern about the aircraft having been stressed beyond limits? E.g. the autopilot development resulted in a full scale control surface deflection that stressed the airframe beyond its g-limits. Restoring the physical attributes would result in the introduction of a non-airworthy (?) aircraft...

No because with a metal structure, for a one time overload, if the structure did not exceed its elastic limits, that is, if it's not permanently bent, it's good to go.  I know of an aircraft with the OEM I used to work for (not Boeing) that was overstressed on a production test flight because of some mishandling, and exceeded the technical negative G limits, but nothing was bent out of shape and the airplane was delivered to the customer after some inspections (and I'm pretty sure the customer was informed and given some price relief as compensation).

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What protections does the designation for aeronautical spectrum provide?
Stated differently: As an aviator, what benefits do I receive from the radio frequencies used by my avionics and ATC being designated "aeronautical protected spectrum" as opposed to just regular licensed spectrum?
I understand that international harmonization around what is considered aeronautical protected spectrum occurs at ICAO. I also understand that in the United States (and most other countries), radio frequency spectrum is considered a public good, and thus owned by the state. Users of spectrum (e.g. cell phone service providers) receive licenses to use a certain part of the spectrum, but ownership remains with the government.
What I don't understand is what happens when a spectrum band is considered "Aeronautical Protected Spectrum:

Does the designation mean it can't be licensed to a private entity
(i.e. it has to be used by the government for purposes of providing
aeronautical services)?
If it can be licensed, how is that licensing
different than a license that a cell phone provider would receive for
their spectrum band?

EDIT: I understand the benefits of being protected vs unprotected; see Michael's answer. Cell phone spectrum is licensed to the cell phone company and unlawful broadcasting in that spectrum is prosecuted as well, but it isn't aeronautical specturm.

Note: public good doesn't mean owned by public authorities. Just that when there are not enough resources for all, restrictions are applicable. E.g. on very high frequencies (as generic term, not just VHF) were free until there were need to use them (and the top part is still free). In any case, it is just notation: FCC do not uses such notation: https://transition.fcc.gov/oet/spectrum/table/fcctable.pdf (page 21). For me it is more for aviation only and not just also for aviation (and possibly: internationally decided, not by single countries).

In this context, "protected" likely means "can claim protection from harmful interferences from other users", this is related to band sharing. This is possible only when the band is allocated by ITU as a primary user. Allocation is done on a regional basis. E.g. other users cannot claim protection in this weather radar band shared with Earth exploration satellites, but see also, e.g. footnotes page 7-44.

"in the United States (and most other countries), radio frequency spectrum is considered a public good, and thus owned by the state". Radio frequencies are solely allocated by the UN, namely its ITU agency, and do not belong to anyone. ITU members agree on the allocation of frequencies on WRC conferences each three/four years. In the US FCC manages the spectrum allocated by ITU to the US.

Aeronautical spectrum must be used for aeronautical uses. Specific slices of aeronautical spectrum must be used for specific aeronautical uses. Only those licensed to do so are permitted to use part of the spectrum.
This is just like any other protected part of the spectrum. There's nothing special about it.

"Aeronautical spectrum must be used for aeronautical uses", there is no absolute "aeronautical spectrum", some bands allocated to aeronautical services are also allocated to other users. Protection can be claimed or not, depending on the status of the users. Bands are also allocated on a regional basis, not on a WW basis.

@mins I don't mean to imply that no bands used for aeronautical purposes have other allowed uses. My point is that aeronautical spectrum is not special at all- just like any band of spectrum it can be used for allowed purposes and not for others.


text:
Does accepting a VFR Descent cancel IFR?
After flying on an IFR clearance through a line of Thunderstorms, ATC provided me the option of a "VFR descent at own discretion". After descending and with the field in sight I requested to cancel IFR, to which her reply was that "you've been VFR this whole time".
My understanding was that a IFR has to be canceled explicitly by ATC (as discussed in this question), although I can't find the reference in the ATC manual (7110.65).
So my questions are:

Does ATC need to explicitly state "Cancel IFR" for a flight to transition to VFR?
If so, where is this requirement stated?

Not sure what link you are referring to (there was none in the original post), but I've added the link to the FAA splash page.

Once you have accepted a maneuver specifically described as under Visual Flight Rules why would you think you are still covered by Instrument Flight Rules?

Bingo, what DJClayworth said.  While you may not have specifically said the words "cancel IFR", whatever words you chose to use when accepting a VFR descent apparently accomplished the same thing.  You are typically either IFR or VFR, so if you accept one you should not presume to simultaneously be the other.

@DJClayworth There's at least one example: despite the name, VFR-on-top is an IFR clearance.

Whatever answers you get here, I'd suggest you check liveatc.net to see if you can find a recording of your conversation with the controller. Since controllers can't cancel IFR it seems like there may be something that was missed or misunderstood in the exchange.

@Pondlife - Interesting, controllers can't cancel IFR? I.e. it has to be initiated by the pilot at all times?

@nodapic Yes, see the ATC Orders 2-1-4: "It is solely the pilot’s prerogative to cancel an IFR flight plan." A controller can't cancel because they have no way to know if it would be safe/legal, and what the operational impact would be on the flight. Maybe the pilot's in IMC, or about to enter IMC, or must be IFR per their company's OPSPECs, or whatever.

Paragraph 7–1–2 of the 7110.65 permits a controller to issue a "clearance" to maintain "VFR conditions" if an IFR-flight-plan pilot requests a VFR climb or descent. The paragraph goes on to say that an "alternative clearance" must be issued if there is reason to believe the pilot cannot maintain VFR.
Remember that VFR is not the same as VMC; there are rules pertaining to cloud clearance that you must follow if told to maintain "VFR."
But of course the Note at paragraph 2–1–4 is very clear that "It is solely the pilot's prerogative to cancel an IFR flight plan." 4–2–10 prescribes the phraseology to confirm that a pilot has cancelled: "(Call sign) IFR CANCELLATION RECEIVED." If you did not hear that, you can assume your IFR flight plan is still active. Reading between the lines of the phraseology prescribed at 7–1–2, my interpretation is that an IFR pilot cleared to "maintain VFR conditions" does not receive IFR separation for a period of time1 but may subsequently be instructed to maintain a set altitude and/or heading and would from that point receive IFR separation again without needing to hear "Cleared to [clearance limit] via..." As I read it, the act of requesting a VFR climb or descent does not necessarily imply a request to cancel IFR altogether.
So to sum up: My opinion of the rule is that the controller was correct, you had been VFR since being issued the VFR descent and you were not receiving IFR separation. But you were still on an IFR flight plan, and that plan had not been canceled yet. The situation was probably muddled by your being close to your destination airport.
If I was the controller I would want to hear you say, whether before or after the VFR descent, "N12345 cancel IFR." If I switched you to CTAF before then I would issue "Report cancellation of IFR in the air this frequency or on the ground at [other frequency]."

1Supporting evidence that a pilot cleared for a VFR climb/descent does not receive IFR separation is found at paragraph 13–2–2a8(a)(9) which deals with the "Conflict Probe" tool used to ensure separation in the oceanic environment. The tool does not account for, among other things, separation provided by using the procedures of paragraph 8–8–5, VFR climb/descent. Of course, VFR pilots are separated from other pilots by the "see-and-avoid" rule, which the Conflict Probe cannot anticipate. But note also that the tool still tracks these aircraft, meaning their flight plans are not cancelled altogether.

Good answer, and I typed my comment on the question above before reading this.  It does raise some questions though, first and foremost; why might a controller choose to use a potentially ambiguous statement like "maintain VFR conditions" versus something more specific and clear like "deviations +/- 20 degrees approved to maintain VMC, report back on course."?  We should all understand the difference between VFR and VMC, (C = Conditions...) why muddy the waters mixing terms?

@Michael I've never used this but it could be the case that the controller is unable to approve a deviation based on other IFR traffic. I could come up with some scenarios; for example a pilot wants to avoid a thunderhead but retain their IFR clearance in order to get below a cloud layer farther along their route. If the controller can't approve deviations the pilot could request a VFR descent to get below the traffic. Perhaps.

Thank you @randomhead, great answer. That was my mental model; I had not explicitly stated "cancel IFR", but only accepted the descent. When I did explicitly state "request cancelation of IFR" is when she implied that my flight plan had already be canceled. So that was the confusion that lead to the question posted here in the first place... Thank you!

At KLAR, it was common for Lakes Air B190s to request "VFR climb-on-course", because the departure procedure sends you to the VOR to climb-in-hold.
The phraseology was "climb in VFR conditions, maintain one-five thousand" (their filed altitude), and we'd advise them of any traffic they might be climbing through.
Conversely, if I had multiple IFR targets overflying an airport, and an aircraft coming from a higher altitude that wants to land, I could advise the landing aircraft of the traffic, and inform them to expect lower when clear. If the aircraft then requests a VFR descent, the phraseology is "descend in VFR conditions through XXX." (altitude 1000 feet below the lowest traffic, but at or above minimum IFR)
You're still on an IFR clearance, but in "VMC+see-and-avoid" until the IFR altitude is reached.
In the US, this clearance cannot be used if any part of the restriction is in Class A airspace.


text:
What requirements are there for radio systems in aircraft?
What are the requirements for radio systems in aircraft? For example, are there requirements about:

Reception ability
Transmit ability
Transmit power

This is primarily about voice communication systems, but information about other radio systems is welcome as well.

What radio systems are you referring to? Voice communication or those for navigation / surveillance / data comms as well?

There is a big difference in requirements between airspace classes, and between flight rules used. As emphasized by vortaq7, no radio are mandatory for VFR in E and F airspaces in the US. But as soon as IFR is used, or A, B, C or D airspaces are entered, there are requirements for radio equipments (duplex voice, navigation, ILS, transponder, TCAS, SELCAL, GNSS, ELT)

What are the requirements for radio systems in aircraft?
Generally there aren't any, at least as far as "being required to have radios".
It is perfectly legal to fly around in most of the United States in a Piper Cub with no radios of any kind.
Indeed, unless it's been retrofitted with one a Piper Cub would have no electrical system at all - so you would only be able to fly under "daytime VFR conditions" in airspace not requiring communication with air traffic control (absent special arrangements).

If you elect to fly an aircraft equipped with radios (as most of us do) there are specific requirements those radios must meet, which are described in Technical Standard Orders (TSOs).
This is not a comprehensive list, but:

For Communication radios these are outlined in AC 90-50D and TSO C-169a.
Transponders must meet specific TSO performance criteria per FAR 91.215
Navigation Radios (VOR/ILS/ADF) must meet specific TSO performance criteria if used under instrument flight rules.
The TSO number for these escape me at the moment but I'm sure you can look them up.
GPS systems must meet TSO C145c / TSO C146c requirements (if used for IFR navigation).
The Emergency Locator Transmitter (ELT) must meet specific TSO criteria.

The requirements obviously vary based on the type of radio, and typically reference other industry standards (for example TSO C-169a for VHF communication radios sends you over to RTCA/DO-186B for the actual performance criteria).

Transport Category aircraft certificated under 14 CFR Part  25 do have requirements for radios per §25.1307 Miscellaneous equipment. (d) Two systems for two-way radio communications... and (e) Two systems for radio navigation...

There are differences also between if the airplane is Experimental, or Certificated. Equipment that is marketed for Experimantal aircraft may not be installed in Certified airplanes - for Certified, there are additional steps taken by the manufacturer to satisfy the FAA. This can be seen at the OEM websites, and by the resellers.
Garmin G5 for example:
Experimental price 1199 USD
Certified price 2149 USD
Transponders must be permanantly mounted and ADS-B Compliant by Jan 1, 2020 for pretty much any plane that needs to have radio communications. Originally non-electrically equipped (old tail draggers for example) are exempt,  but than cannot fly into radio required regions without prior coordination.
FAR 91.225
FAR 91.227

Can you expand on this and add some sources? What exactly are the differences between experimental and non-experimental in this case, for example?

I don't know, one would have to inquire with the OEM.


text:
Can one make an electric-sailplane from transparent carbon fiber?
Is it technically feasible to make a sailplane from transparent carbon fiber? I am looking for an answer based on the specifications' comparison of the 2 different fibers.
And can it be made self-launch capable with an electric motor ? ie. the difference in the design considerations while positioning and designing an electric motor for a normal glider vs a carbon fiber glider; Eg. Sparrowhawk.

Sorry to disappoint,  but the product you are referring to is just a fake stick-on plastic laminate with no fibre content or structural use whatsoever. Carbon fibers (whatever type)  are inherently black (in the visual spectrum, X-ray e.g. is different though).

@yankeekilo Something similar could probably be made from a thin layer of fiberglass.

Sure,  but any structurally useful amount of carbon will be opaque. And completely transparent structural  GFRP (with suitably high fiber content) is not yet available.

Carbon fiber is widely used for sailplanes in any case already,  a lot of them motorized. But they are all quite non-transparent (but gorgeous nevertheless).

Is there a list of electric carbon-fiber gliders. And what is the difference between GFRP and CFRP in terms of structural integrity and the layering process.

I would recommend asking specific questions,  right now this one is too broad. Wikipedia also has any amount of info you should browse to prep your knowledge. For electronic gliders,  look up Antares or e-Genius for example.

Yes, sure. Although the Antares is made aluminum and so is the e-Genius.

Most assuredly not - both make heavy (pun intended) use of CFRP. But I meant this Antares and this e-Genius :D  Note: If you want to message someone, add a @username in the comment, then username gets noticed.

Please see the link below. Some gliders were upgraded/had a carbon fibre variant. The carbon fibre would be far stiffer than the glass fibre, resulting in a higher performance (especially at speed), or indeed in any scenario in which the wings flex. As the wings flex you're altering the angle of attack which in turn could cause more drag reducing the performance of the glider. Please read the wikipedia page, the Nimbus 2c was upgraded to a partial carbon fibre construction.
http://en.wikipedia.org/wiki/Schempp-Hirth_Nimbus-2

@yankeekilo So, with the current state of technology, there cannot be an sailplane with its fuselage made of GFRP, let alone mounting an electric motor on it ?

The motor can be built into any kind of fuselage, only optically clear GFRP is not available.

I am not entirely sure what the main aspect of you question is (transparency? motor integration?...)- perhaps you could rephrase it a bit to make that clearer (at the moment the Q is too broad).

@yankeekilo The empty weight of the Sparrowhawk is 70Kg. That is the lightest glider I know of. I was looking for a electric glider with a CRPF fuselage for lightweight. I thought of transparency because I thought mechanical fault identification\diagnosis would be considerably easier with a transparent structure.

As long as there are load-bearing structures beneath the fiber (I don't know how strong that fiber is so I will assume it is fabric-like), it will most likely fly if designed properly. If the fiber is smooth, it shouldn't be much different than regular fabric or other composite skins.
And yes, there are plenty of sailplanes that self launch with electric or gas motors. You would just have to take into account the weight and position of the motor when you add it in. This could mean that you need to lengthen the tail section for more tail down force, move the seats further back, place the engine in a more neutral CG position above the fuselage on a pylon, or any number of other methods. You might then also need to increase the wing surface area a bit, to lift the extra weight. Also take into account the weight and position of the batteries. The design effects and considerations are pretty straightforward, but dealing with them may or may not be complicated.
The Soviets tried in the 1930's to make an invisible spy plane (an AIR-6 I believe) by replacing the cloth skin with a cellophane-like material so that its entire was transparent. At just a few thousand feet the engine could be heard but the plane was almost completely invisible. Unfortunately the cellophane-like material would rapidly degrade after a few flights or several days out in the sun. It seems like your idea would be the modern version of that endeavor.

That plane used acrylic. Carbon fiber is a far superior material. And that would give additional structural benefits.
The Sparrowhawk is a monocoque structure.

But all carbon fiber monocoque structures that I know of are composites, layers of fiber with epoxy between them. I would think that each layer of material and epoxy would make your skin more opaque, and the clear fiber you're asking about isn't all that transparent to begin with. So yes, you could certainly build a monocoque fuselage with the transparent fiber as long as it is strong enough and you layer it correctly. But I think you would lose most of your transparency in the process.


text:
How do airlines dispose of a passenger jet when it's no longer airworthy?
When a plane is finally considered to not be airworthy, what happens to them?  Do they take them apart bit by bit?  Do they sink them offshore to make them into reefs?  Are parts stripped and sold as spares? Are there legal procedures the airlines have to go through?  What happens?

First, they fly it to a boneyard such as this one in Mojave, CA.

Sinking anything to create reefs is highly controversial - even most offshore drilling platforms (where reefing has proven ecological advantages) are brought onshore and cut up for scrap.

See also this question: How do i get permission to fly a plane that isn't airworthy

I'm not sure a detailed answer is really going to be all that helpful since the answer is they go wherever someone is willing to buy them - engines, tires, etc.  If someone wants to buy tires, guess what, they take the tires off without destroying them.  However, for most consumable parts getting the proper paper trail on them probably isn't worth it, so they're just scrapped.  The high-value avionics, engines, seats, etc are sold to whoever gives the most money.

@Aaron Perhaps, but there's 100 points in it for anyone who's willing to make a good synopsis of the process.

If you are frenchspeaking, there is C'est pas sorcier dealing with this subject (Que fait-on des vieux avions?).

@ManuH Sadly no, English and some Japanese...  If you want to summarize it in English here though...  There may be 100 points in it for you.

@GregHewgill I doubt all planes visit boneyards like that, in which case, it's misleading to say that's the first step.

@DavidRicherby: My comment is clearly not intended to be comprehensive. Mostly I just wanted to include a link to a pic of that boneyard because it's pretty cool and it might be helpful for somebody who hasn't seen it yet.

Generally the planes are scrapped by a recycling company. The parts will either be removed and sold as spares, or chopped up and recycled or disposed of. So a lot will depend on how easy it is to remove the part, and how much of a market there is for it. Airplanes are disassembled to a large extent during heavy checks, which can provide an opportunity for substituting in spares.
Engines: Probably the most valuable part of the airplane. These will be removed and either used as whole spares or for spare parts, depending on the condition and how many of the type are still in service.
Cockpit instruments: These will be removed and used as spares as well, if there is enough of a market (everyone wants those fancy glass cockpits these days).
Interiors: These may be removed as well, they can be used as spares.
Control surfaces: These can be removed and used as spares as well.
Windows: These can be removed and reused.
Systems: Pumps, electronics, lights, actuators, etc.: Most of these can be removed and sold as spares.
Landing gear: This can also be removed and reused, depending on the number of cycles it already has.
Tires: The tires may be reused if not worn out. The tubes may not be reused.
Airframe: Although some parts may be able to be used, generally the effort and the fact that there are probably many cycles on the part will prevent much from being used as-is. The metal will be chopped up as scrap. They are already working on recycling major composite structure as well, since manufacturing scrap is already being created. According to Boeing, with modern recycling processes, the metal, plastic, wires/electronics, and carbon fiber composites are used for:

15% High-grade industrial
50% Low-grade industrial
35% Landfill

As it often is with business charts, it's not clear if these are by weight, volume, or what. This company claims to recycle 90% of the weight of an aircraft.
There are of course cases like Steve V. mentions where the plane is donated to some institution, but this doesn't happen to most planes. There are also ways that airplane parts find other uses.

I wonder if you can comment on how spares are sold?  Is there a secondary market place that has a few distributors or are airlines selling direct?

@JayCarr: My last flight with Kingfisher was in a fairly new A-320 in very old chairs. I can't prove it but am sure those chairs had survived their first aircraft. Note that furnishings normally are replaced more frequently than the jets they fly in.

They send it to a professional demo team that drains the fluids, grabs the spares and cuts up the hull for the materials and send it all to be recycled as possible.
There are plenty of youtube videos detailing the process like https://www.youtube.com/watch?v=xDxJAO__6t8

Are there any rules regarding what they are allowed to pull off the planes?

@JayCarr that'd depend on the contract they have with the airline. If it's like with cars to be scrapped, usually either the contract states which bits to be returned to the (prior) owner, or everything becomes property of the wrecker. The idea is to leave nothing, reuse/resell what you can and dump the rest (as little as possible) according to local law (chemical/radiological/etc. waste regulations can take effect).

FedEx has lately been donating their 727 retired fleet to flight schools and training departments.
FedEx Donates Boeing 727 to Trinidad & Tobago Civil Aviation Authority
FedEx Donates Retired Boeing 727 To Broward Aviation Dept

Very few end up in a career as firefighter training objects.


text:
Under what conditions do airliners retire aircraft and sell them to other airlines?
How often (and under what conditions) are airliners resold to another organization with...less stringent qualifications?  Is there a certain number of hours after which an airliner has to be retired or something?  Or is it just that airlines like to buy new and shinier airplanes on occasion?
Further, when an aircraft is being resold, are there any particular procedures that have to be followed?  And are there different levels of airworthiness involved (ie., you have a lower litmus test if you are just selling it off to be used as a cargo plane or private plane?)

Some reasons for an airline to sell some of its fleet:

they need quick cash (as ratchet freak points out). This usually means the company is in serious trouble
they're changing their route network and there's no more need for it (e.g. KLM dropped a lot of very short flights in the 1990s and early 2000s, and got rid of the aircraft that were operating them as they were inappropriate for the longer routes that replaced them)
they bought out another airline and have no need for (all) its aircraft, or want to replace them with other types to keep their fleet homogenous (operating many similar types is less economical than operating a smaller number of types)
they're splitting part of their operations to a new company set up for the purpose (the new company becoming the new owner of the aircraft, usually for a symbolic fee), for example a regional affiliate or a charter operation
it may be more economical to replace the aircraft then to keep them. When NWA decided to replace their DC9 fleet with A320s this was for economic reasons (lower maintenance and fuel cost for example), not because the aircraft had run out of fatigue life. The aircraft were old so most probably were scrapped, but some may well have ended up with airlines in other countries, replacing even older aircraft or expanding existing DC9 fleets there.
and some airlines just want to portray a public image of always flying shiny new aircraft. I believe Singapore Airlines used to have a policy like that and replace everything it bought relatively quickly (more so than most other airlines at least). PanAm was like that as well. If you have a reputation of exclusivity, this may add to customers' willingness to pay premium to fly with you

+1 Lots of good points, including the fact that there are many good reasons to do it. Another one you could add is keeping maintenance simple. Many low cost carrier deliberately choose to fly only new aircrafts, all of the same type, replacing them quickly and occasionally switching from one type to the next. They are therefore constantly selling perfectly good aircrafts as part of their strategy.

@Relaxed yes, good point. And of course many of them will just take relatively short dry lease contracts on their aircraft, returning them to the lease company for the newest latest and greatest after 5 years or so

Answer to this question can be very conditional. Different airlines have different policies and depends on economic status of country its operating.

Some airlines have policy to retire aircraft after a fixed number of hours of operations irrespective of the condition of aircraft. They either sell it to other company our lease it out. This model of aircraft retirement prevails in company having robust operation and good flow of finances. Under these conditions retired aircraft are not UN-airworthy or unfit for flying,however they are used by small airlines and also in less developed countries.
Second Consideration on retirement of aircraft, also depends on pressurization cycles that an aircraft underwent. In big transport airlines aircraft there is pressurization system , which involves cycle of pressurization. Each cycle has impact on air- frame of aircraft. After certain number of pressurization cycles aircraft may be assume to be unfit for flying and can be retired.
Third scenario is aircraft met an serious accident and its beyond repair , hence declared un-airworthy, therefore retired.
Aircraft must meet certain parameters in terms of performance, if aircraft does not meet that criterion of performance set by civil aviation regulation authority, in this situation an aircraft has to be repaired to achieve required standard of airworthiness. In case if it cannot achieve that hence again it is retired aircraft.
Time to time aircraft is inspected by civil aviation regulatory authority for the health of its components, including air frame , engines etc. In situation aircraft is declared to crossed it age to be airworthy , can also be treated as retired aircraft.
Most common criterion of aircraft retirement is fixed by air regulation authority , if they declare aircraft un-airworthy and deny to issue certificate of airworthiness. This is ultimate situation of aircraft retirement

To answer your second question, when aircraft are being sold it is common practice to perform a check ride in which the plane is put through a series of less-than-normal flight maneuvers that would reveal possible signs of impending maintenance and/or damage. This (AFAIK) is not a requirement, but rather a good business practice.
To answer your first question (poorly), it most likely depends on airline policies. Aircraft that are not fit for flight are simply not fit for flight, however airline policies, and/or the economics of retaining specific aircraft may contribute to the sale of airliners. An additional factor may be fluctuations in demand for certain routes, in which cases certain types of aircraft may become less feasible to own (turboprops are more economical for shorter routes, jets are better for longer routes).
Airlines also engage in the practice of loaning aircraft to other airlines.
Source: http://airinsight.com/2011/08/19/jet-vs-turboprop-a-debate-that-dates-from-the-early-1950s/#.U_WMD7xdUkU

turbofans are better for long routes only specialist applications use turbojets anymore

@BrianWheeler it's quite possible for an aircraft to be unfit for service in one country but legal in another. Especially in Africa and parts of Asia the maintenance and safety requirements are rather more lax than in say Europe or north America.

@jwenting Are you sure requirements are generally more lax in Africa? I would suspect that it's mostly enforcement which is lax or even completely lacking in some countries…

@Relaxed a rule that doesn't get enforced is a rule that might as well not exist... So whether it's de-facto or de-jure, the rules are more lax there (and for sure that's the case for emission and noise requirements, which is a big reason for European and North American airlines to retire aircraft before they run out of fatigue life).

There is a maximum number of flights/flight hours after which the plane needs to retire (aka scrapped for parts). The main reason for this is the metal fatigue by the pressurization cycles. If you ignore that then you have the same issue as Aloha flight 243 meaning the fuselage will simply rip itself apart.
One reason an airline sells a plane is if it is strapped for money. And no company is going to buy a used plane when they won't get their investment back.

Airlines have many reasons to sell aircraft, needing the cash is just one of them.

Money is a common factor, but cash demand is just one concern. A company which retired 9 out of 10 planes of a given model may sell the last one so they don't need to stock spare parts anymore, keep pilots proficient on the model etc.

“The only reason” is inaccurate.

There are few reasons an airline would sell an airworthy aircraft. As long as the aircraft can fly, it can make money. It typically comes down to pure cost-benefits and risk analysis:

First off, if the FAA says the plane's flying days are over, then there's no decision to make; it makes one last run with a flight crew only to a boneyard like Arizona, where it will eventually be stripped for usable parts and the rest scrapped to recycle the metal. Even if the FAA will let the plane fly, the airline's insurer may take a look at the maintenance history and the problems found and say "uh-uh", figuring more problems are just around the corner.

Second, even if the frame's still airworthy and insurable, it might cost more in replacement parts and maintenance to keep it that way than it's worth carrying passengers. AA finally came to that conclusion with its MD-80 fleet; the Super-80 has been out of production for over 15 years now, parts only get scarcer, and its relatively narrow fuselage increases load-in/load-out times and reduces passenger comfort compared to its replacements like the A319 and 737 (which carry about the same passenger count in a shorter, fatter fuselage that feels roomier), meaning people were less and less likely to choose American if the plane for the flight would be a Super-80.

Third, an airline's basic strategy may change. An airline may spin off a regional subsidiary (and its Embraer puddle-hoppers), or reduce international flights (thus retiring some older 747s without replacing them). Southwest is making a more subtle change, opting for the larger 737-800 over the 300s and then 700s it had previously favored, using the larger size to get more passengers into fewer gates at the smaller or older airports it tends to service, like its primary hub at DAL which is limited to 20 total gates by the agreement that repealed the Wright Amendment.


text:
How much pollution is created by an airplane taking off?
I read on a site that a 747 taking off emits the same amount of air pollution in the first 5 miles or so as 3000 cars. This seems extreme (accepting that the cars will be average).
Are there any more precise comparisons for 2 or 4 engine aircraft?

As 3000 cars would in 5 miles, or in the time that it takes the 747 to travel 5 miles? Driving uphill like the 747, or on a flat road? If you are not precise in your question then the answer definitely is: yes there are more precise comparisons.

I've no idea if the number is extreme or not but remember that there are fewer than 1,500 Boeing 747s in the world but a couple of orders of magnitude more than 1,500x3,000 = 4.5 million cars.

@DavidRicherby Boeing just delivered the 1500th 747. So you don't have to estimate as your calculation is completely accurate now.

@farhan don't forget to account for the retired/lost hulls

@ratchetfreak I agree: 1500-52=1448. But didn't they already generated their pollution?

Is that a fair comparison?  Measuring the 747 at what is admittedly the worst case condition - on takeoff - to automobiles in cruise mode?  Put those 3000 cars on a drag strip and measure again...  And then break down the answer to compare pollution per occupied seat mile.  This sounds like a statistic created to serve an agenda.

Where did you read this? (It's generally good form to cite your source - An old press hack once told me "Never respond to a blind quote.")

@SkipMiller - that's how you do it if you're trying to prove a point. My best against your worst - there is no such thing as a fair fight.

@FreeMan:  So where does it say Aviation Stack Exchange is a battle board?

There's any SE site where can I post a pollution comparation between a bus commuting people to work and a RC model?

Its all about the definition of "pollution" used. Jet A has higher quantities of certain by-products that are considered pollutants than does automotive gasoline. For example, pollutants can be considered to be one, some or all of these: hydrocarbons, nitrogen oxides, carbon monoxide, sulfur dioxide, or other trace toxins. If the OP's article was referring to sulfur dioxide (SO2), then the claim might be true, because gasoline engines emit almost none of these, but diesel (JetA) produce much more. If this is the case, the claim is very misleading.

To provide more detail, let's look at some actual numbers. Without knowing where you got this information, the term "air pollution" is especially vague. But let's see what we can do. I'll compare CO (carbon monoxide) emissions since I could find that for both cars and jet engines.
The new 747-8 uses the GEnx-2B67 engine. The older 747-400 has a few engine options, one of which is the CF6-80C2B5F. Emissions information about these engines is available from EASA tests, and available for the GEnx and CF6. More info about the tests is here.
Based on that information the two engines have the following CO emissions at takeoff, reported as grams per kg of fuel burned:

CF6: .05 g/kg CO
GEnx: .17 g/kg CO

The fuel burn rates at takeoff for the engines are reported as:

CF6: 2.685 kg/s fuel
GEnx: 2.451 kg/s fuel

To find a value of comparison for cars, the EPA standards are here, in units of g/mi. Let's assume that we are using a recent model year, certified to the ULEV (Ultra-Low Emission Vehicles) standard.

ULEV: 1.7 g/mi CO

Now we need a way to compare these values. Let's use your initial value of 5 miles as the comparison distance. The jet engines will remain at takeoff thrust over these 5 miles, and the cars will meet the ULEV standard.
So how long will it take the 747 to cover those 5 miles? Let's look at the whole takeoff run, from a complete stop. This means that it will take about 40 seconds to cover the first mile down the runway. After that, lets assume it averages 160 knots, or 185 mph, over the next 4 miles. This means it will take 78 seconds to cover the rest of the distance, for a total of 118 seconds.
Computing the emissions for the takeoff:
CF6:
$$\begin{align*}
2.685\,\mathrm{kg/s} \times 0.05\,\mathrm{g/kg} &= 0.13\,\mathrm{g/s}\text{ CO}\\
0.13\,\mathrm{g/s}\times 118\,\mathrm{s} &= 15.84\,\mathrm{g} \text{ CO per engine}\\
15.84\,\mathrm{g}\times 4\text{ engines} &= 63.4\,\mathrm{g}\text{  CO per takeoff.}\end{align*}$$
GEnx:
$$\begin{align*}
2.451\,\mathrm{kg/s} \times 0.17\,\mathrm{g/kg} &= 0.42\,\mathrm{g/s}\text{ CO}\\
0.42\,\mathrm{g/s}\times 118\,\mathrm{s} &= 49.17\,\mathrm{g}\text{ CO per engine}\\
49.17\,\mathrm{g}\times 4\text{ engines} &= 196.7\,\mathrm{g}\text{ CO per takeoff.}\end{align*}$$
The car will be easier to compute:
$$5\,\mathrm{mi}\times 1.7\,\mathrm{g/mi} = 8.5\,\mathrm{g}\text{ CO per car.}$$
The 747-400 holds 416 people, and the 747-8 holds 467 (3-class configuration). Let's assume each car will carry 4 people.
$$\begin{align*}
416/4 &= 104\text{ cars}\\
467/4 &= 116.75\text{ cars.}
\end{align*}$$
We'll split the difference between cramming the three extra people into other cars and adding another car, and just leave it as 116.75 cars. Totaling the emissions from all cars:
$$\begin{align*}
104\text{ cars}\times 8.5\,\mathrm{g/car} &= 884\,\mathrm{g}\text{ CO}\\
116.75\text{ cars}\times 8.5\,\mathrm{g/car} &= 992\,\mathrm{g}\text{ CO.}\end{align*}$$
So comparing the 747 and the cars:

747-400: 63.4 g CO
104 cars: 884 g CO
747-8: 197 g CO
116.75 cars: 992 g CO

So a 747-400 is equivalent to about 7.5 cars, and a 747-8 is equivalent to 23.2 cars, comparing the takeoff run with average vehicle emissions.
I found this pretty surprising. It would be more fair to compare a takeoff with a drag race or a 0-60 test on the cars, which is probably not what the emissions test requires.
If anyone has more info or sees an issue with this please let me know.

It would be more fair to compare a takeoff with a drag race -- It would probably be more fair to compare the total carbon output per passenger-mile for the entire flight (takeoff, cruise, landing) with driving equivalent distances.  This is more important when comparing mileage figures (the EPA Miles-per-Gallon figure for cars are averaged and include time/fuel spent on acceleration (takeoff), cruise, and deceleration (descent/landing)).

I agree that comparing per passenger mile is much more relevant, but based on the question I tried to focus on takeoff alone.

It seems really weird that the GEnx produces 3x as much CO as the older CF6... any idea why that would be?

So to put it in the relation provided in the question, you don't end up with 3000 cars but 7.5 and 23.1 cars, respectively? I've yet to see a highway where the total average number of occupants per car was higher than (or even close to) 2. Empty car-pool lane during rush hour anyone?

@David I agree that is strange. Maybe it was a compromise to reduce the HC and NOx, possibly related to the TAPS combustor.

@David given the current fashion to optimise everything for CO2 emissions at the expense of everything else, it's not surprising. The CO2 emissions of the two are probably the reverse of what you see here. Unintended side effect of the AGW hoax is that real pollution increases for the sake of reducing harmless CO2 emissions... You see the same in car engines, many now produce more NOx and CO per liter of fuel burned than their predecessors. For both car and aircraft engines, total fuel use (the real thing to consider) is typically lower for newer engines.

Well, CO₂ emissions are even easier to compare. They are fuel burn rate times appropriate factor depending on fuel used. Because all carbon in the produced CO₂ comes from the fuel and majority of carbon in the fuel is oxidized to CO₂.

Just looking at CO is misleading. Aircraft gas turbines run very lean, so much less CO per mass of fuel burnt is produced than in a car engine. If you need $CO_2$ figures for a more complete picture, take the used fuel mass as a proxy. If you add unburnt hydrocarbons, cars with a catalytic converter will come out so far ahead, it is embarrassing.

While there are more pollutants than CO, but keeping with CO, if we use 1000kg fuel as typical for a 747 taxi and takeoff, and compare that with 2 miles of automobile driving, I get the CO per passenger is much smaller for the 747:
747
0.05    fuel CO g/kg;
1000    Fuel used in Taxi-TO (kg);
50  CO produced (g);
400 Passengers;
0.125   g CO/passenger;
CAR
9.4 CO g/mile;
2   miles;
18.8    CO Produced (g);
4   Passengers;
4.7 g CO/passenger;
So, the CO produced per passenger is about 40 times greater for an automobile.

Be careful about comparing apples with oranges! The '40 times greater' refers to the amount of CO pollutant produced per passenger comparing generic fuel consumption for a 747 between taxi and takeoff, compared to a car travelling 2 miles. I'd have no heartburn with arguing it would be fairer to have the car travel 10 miles, in which case the ratio would be almost 200:1. None of this can be easily interpreted as statements of engine efficiency.

Just looking at CO is misleading. At least add $CO_2$ for a more complete picture. Aircraft gas turbines run very lean, so much less CO per mass of fuel burnt is produced than in a car engine.

Uff way to complicated .
747-400 fully loaded 160t of fuel (can take more fuel). 160.000 x 0.87 (fuel index) 140.000 liters approx.
140.000 L / 50 (average tank full per car)= 2800 tank fulls
140.000 gives probably 12 to 14 k nautical miles.
One car filled up 2800 times = about 2 million km. At about 7l/100 km.
This has nothing to do with mile per pax etc. As rarely do aircraft fly fully loaded or even with pax as many cargo aircraft in the air as well.
Just thought I make it a little bit more easy to imagine.

Welcome to aviation.SE! This is relevant, but the question was about pollution, not fuel consumption. If you can expand your answer to include that, it would be more useful.

@Pondlife one could pose that amount of fuel pollution iis proportional to amount of fuel used.

@Koyovis Sure, but I personally don't know if that's a reasonable conclusion. It's like asking "I heard that one hamburger has 5 times the calories of a hotdog, is that true?" It sounds like it should be, but if you load up the hotdogs with chili and cheese and compare them to a tofu burger, maybe not. And now I'm hungry :-)

@Pondlife Good point, how much does he eat and how good is the food.


text:
What airports close to the east coast of the US have attractions that are easy to get to?
I'm currently living in Boston and fly out of KBED regularly. I'm looking into making a long cross country in a Cessna 182T to the Bahamas and am planning my route. I'm IFR rated.
I'm looking for airports generally along the east coast (east of KATL and south of KBED) that have at least one of the following and are less than a 25 minute drive from the field: diner/restaurant with at least 3 stars on yelp, museum, historic or tourist attractions.
Some places that have caught my attention:

KBID (Block Island State, RI) - Bethany's Airport Diner is on the
field and has 4 stars on yelp
0B8 (Fishers Island, NY) - friend says
it's a 15 minute walk to a really nice golf club open to anyone
KMYR (Myrtle Beach, SC) - there's a 100 things to do walking distance from the airport (parks, beach, restaurants, tours, etc...)
KCHS or KJZI (Charleston, SC) - 20 minute drive to the historic downtown of the city

Really appreciate your input!

@raptortech97 I've made some edits, do you think I've removed the "opinion-based" problem?

Yeah, that looks great!

I'm voting to close this question as off-topic because it seems to be a topic of Travel.SE.

My first thought was: Old Rhinebeck!. But I guess you are looking for something completely different.

@kevin Although I think even Travel.SE generally discourages "What stuff is there to do in X place?" questions, since list-type questions generally don't fit the SE format well.

Asking for good (GA) airports to visit would be more on topic here than Travel.SE. I think reirab's point about list-type questions is more relevant, as well as the location of "east coast" being far too broad.


text:
Why is the aircraft boneyard in Tucson, Arizona still active?
I understand that an aircraft boneyard is an amazing tourist attraction for aviation enthusiasts.
However, the number of planes that the boneyard in Tucson has accumulated today is mind-boggling. I know that, initially, the purpose of the boneyard was to salvage materials from old and obsolete aircraft. However, the pace at which most aircraft are being scrapped is very low. Though it serves as a spectacular sight, why is the boneyard still active?

Short version: because aircraft are still being built. Also, a lot of the aircraft there are being stored for later resale, not awaiting scrapping.

If the number of planes waiting on getting gutted isn't decreasing even though scrapping activities are happening that means there is a steady influx of end-of-life planes arriving to get scrapped.
As long as there are planes and new planes being built (most have a lifespan of a few decades) there will be planes that need to get scrapped.
You can't just dump the plane on a landfill or you will get nature activists on your case about the hazardous materials (hydraulic fluid, remnant fuel, ...) and recycle potential. Plus most components (instruments, engines, the espresso machine in the galley, etc) will still be usable and airliners will want to buy them as spares.

For that matter, the non-usable components are valuable: a back-of-the-envelope calculation says a Boeing 747-200 is worth around $100,000 just in the scrap value of the aluminum.

Has any actual sale happened from these aircraft boneyard's till date, either of a spare part or an entire plane?

As ratchet freak noted, the boneyard is still active because planes are still being scrapped. One reason specific to the AMARC in Tuscon is the B-52 aircraft still being scrapped for arms reduction treaties. Planes may also be taken for use as targets, training aids, or even to be returned to service. Even if it just seems like the planes are sitting there idle, the boneyard also serves as a huge collection of available spare parts that is continuously being used to supply the aircraft that are still in service.

Typically controlling over 4,200 aircraft as well as many other types of military equipment, AMARG works very hard in promoting itself as not just a 'Boneyard' and takes every opportunity in explaining how it operates its cost effective, tax saving operations. Many of the stored aircraft can be returned to an operational status in a short period of time and there is a continual process of anti-corrosion and re-preservation work which keeps the aircraft in a stable condition during their stay.

So the purpose is not just to scrap planes, but also for storage and reuse. It's not even necessarily just for old planes; it's been used to store brand new planes until a use is found for them.

To tack on to @fooot's answer as far as aircraft reuse is concerned, AMARG also prepares aircraft for potential foreign military sales.  From an FAA circular:

The U.S. military normally retires aircraft to Davis-Monthan Air Force Base in Tucson, Arizona to be stored indefinitely, cannibalized, scrapped, or restored to service at a later date. Surplus U.S. military aircraft are generally not sold directly to private U.S. operators without congressional approval. However, U.S. military aircraft may be exported via foreign military sales programs or other agreements.  (emphasis mine)

This C-130 lived there for three years prior to being sold to Botswana.
Interestingly enough, many aircraft stored there are actually in better and more modern condition than the state in which the US Air Force flew them.  I knew a civilian assigned there who was the back seat test pilot for F-4s after they received the necessary avionics upgrades in order to be attractive to foreign governments.


text:
How to supply a flying club with MOGAS?
I've noticed that there are very few airports that serve MOGAS. If I wanted to start a flying club with a MOGAS powered airplane, but there wasn't an FBO nearby that served MOGAS how would I go about it?

Is it worth it to an FBO to offer MOGAS?
Should I have the FC members supply their own gas with a couple of personal fuel stations like the one pictured below? (anyone have any experience with this method?)

Update: Tried to reword questions to make it less business & more "how-to".

while related to aviation, the question seems to focus on a business logistics problem.

The FC I used to belong to had it's own fuel trailer (custom-built with dual double-walled, insulated cryogenic 100-gallon tanks, a manually-operated fuel pump, a totalizer, a delivery hose and nozzle, and appropriate grounding cables.  Excluding the cost of the basic flat-bed trailer itself (one of the FC members loaned it to the club) we had less than $1500 invested in the outfit - including inspection and trailer licensing fees.
Considering we would be carrying 200 gallons of MOGAS on public highways we had to be VERY careful as to WHEN we were ON the highways (basically very late at night, and we drove on lightly-used roads/streets) - and we had to have insurance commensurate with the commodity we were carrying.  We used this trailer for several years - without any incident(s) - but, in the end, we all felt un-easy about doing so and eventually we abandoned the idea and reverted to using hand-carried gas cans.
The FC disbanded years ago and I don't know what ever happened to the fuel rig - I believe it went back to the FC member who built it in the first place (his materials costs and his labor bought him a club membership, whereas the rest of us had to ante up actual cash to help buy the airplane the FC bought.)
Ahhh, the good ol' days....